In conventional kraft cooking implemented in the 1960-1970-ies in continuous digesters was the total charge of white liquor added to the top of the digester. It soon emerged that the high alkali concentrations established at high cooking temperatures were detrimental for pulp viscosity.
Cooking methods were therefore developed in order to reduce the detrimental high alkali peak concentrations at start of the cook, and thus were split charges of alkali during the cook implemented in cooking methods such as MCC, EMCC, ITC and Lo-Solids cooking.
Other cooking methods were implemented using black liquor impregnation ahead of cooking stages where residual alkali in the black liquor was used to neutralize the wood acidity and to impregnate the chips with sulfide. One such cooking method sold by Metso is Compact Cooking where black liquor with relatively high residual alkali level is withdrawn from earlier phases of the cook and charged to a preceding impregnation stage.
One aspect of alkali consumption during the cooking process, i.e. including impregnation, is that a large part of the alkali consumption is due to the initial neutralization of the wood acidity, and as much as 50-75% of the total alkali consumption is occurring during the neutralization process. Hence, a lot of alkali is needed to be charged to the initial neutralization. This establish a cumbersome problem as high alkali concentrations had been found to be detrimental for pulp viscosity when charged to top of digesters in conventional cooking. One solution to meet the high alkali consumption and necessity to reduce alkali concentration in top of digester was to charge large volumes of alkali treatment liquors, preferably black liquor having a residual alkali content, but having low alkali concentration, which resulted in presence of relatively large amount of total alkali per kg of wood material but still at low alkali concentration.
IN U.S. Pat. No. 7,270,725 (=EP1458927) Metso disclosed a pretreatment stage using polysulfide cooking liquor ahead of black liquor treatment. In this process was the polysulfide treatment liquor drained after the pretreatment stage and before starting the black liquor treatment. The polysulfide treatment stage was also preferably kept short with treatment time in the range 2-10 minutes.
In a recent granted U.S. Pat. No. 7,828,930, is shown an example of a kraft cooking process where 100% of the cooking liquor, in form of polysulfide liquor also named as orange liquor, is charged to top of digester and start of an impregnation stage. Here is also the temperature raised from 60° C. to 120° C. at start of the polysulfide treatment stage. However, as shown in example 1 is a liquor to wood ratio of about 3.5 established in the top of the digester by adding a proper amount of water. This order of liquor/wood ratio is often perceived as a standard liquor/wood ratio in continuous cooking necessary for a steady process. According to this proposal is a part of the residual polysulfide treatment liquor at relative high alkali concentration withdrawn and replaced with cooking liquor at relative low alkali concentration at start of the cooking stage, and the withdrawn residual polysulfide treatment liquor is added at later stages of the cook.
There has thus been an ongoing development of cooking methods where both alkali concentrations at start of cook were reduced, and increased yield from the cooking process is sought for using among others addition of polysulfide cooking liquor that stabilize the carbohydrates.
The present invention is based upon the surprising finding that concentration of polysulfide should be kept high in a low temperature pretreatment stage at relatively long retention time before cooking, using liquor-to-wood ratios (L/W ratios) well below that were commonly used. The stabilization effect of carbohydrates, the major objective for polysulfide addition, has shown to be improved dramatically if using a liquor-to-wood ratio of about 2.9 instead of the conventional liquor to wood ratio of about 3.5, and all other conditions equal. This non-proportional effect of low liquor to wood ratio has not been disclosed or realized before despite the numerous proposals for improving cooking yield using polysulfide cooking liquor.
One object of the present invention is to provide an improved method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor, wherein the lignin-containing cellulosic material is heated to a temperature in the range 50-100° C. followed by adding polysulfide cooking liquor to a first impregnation stage which in turn is followed by cooking stages resulting in a kraft pulp with a kappa number in the interval of 15 to 50, and more preferred in the interval of 17 to 40, and most preferred in the interval of 20 to 40. For some applications, for example when polysulfide cooking liquor is added to a first impregnation step, which is followed by a cooking stage and a refining step, to produce kraft liner, the kappa number can be higher, for example in the interval of 40 to 120, and more preferred in the interval of 70-110, and most preferred in the interval of 80 to 110. The impregnation stage of the improved method of the present invention is conducted at high alkali concentration, low temperature and high polysulfide concentration using polysulfide cooking liquor at a liquor-to-wood ratio in the range 2.0 to 3.2, and that the temperature is between 80-120° C. during a retention time resulting in a h-factor in the range 2-20 and preferably 2-10 of the impregnation stage. This low h-factor is indicative for that no cooking or delignification effect is obtained in the first impregnation stage, and hence is no reduction in pulp viscosity seen as could be the case if high alkali concentrations are at hand in cooking stages at higher temperatures.
According to one preferred embodiment of the method is the effective alkali concentration during the impregnation stage above 60 g/l when adding the polysulfide cooking liquor.
According to another preferred embodiment of the method is the polysulfide concentration during the impregnation stage above 3 g/l, or above 0.09 mol/l, when adding the polysulfide cooking liquor.
According to a further embodiment of the method is more than 90% of the total charge of cooking liquor needed for completion of the cooking stages to the intended kappa number below 40 charged to the first impregnation stage, and that at least 175 kg of alkali (EA as NaOH) per ton of chips is charged for softwood and at least 160 kg of alkali per ton of chips for hardwood.
According to yet another embodiment of the method is the alkali concentration reduced by at least 8 g/l by adding additional cooking liquids having less alkali concentration than the alkali concentration prevailing at end of the first impregnation stage when increasing the temperature to cooking temperature, said cooking liquids in at least part thereof include black liquor.
In a most preferred embodiment of the method is no black liquor added to the first impregnation stage.
When using the inventive method has also preferably the white liquor added to the first impregnation stage an alkali concentration above 100 g/l and a polysulfide concentration above 4 g/l.
The lignin-containing cellulosic materials to be used in the present process are suitably softwood, hardwood, or annual plants.